ti faccio un esempio
	codice:
	<script language="javascript">
function Media(){
	var val,ff=document.NomeForm
	var n=0
	var tot=0
	for(var i=1;i<=4;i++){
		val=ff.elements['NomeText'+i].value
		if(!isNaN(val) && val!=''){
			n++
			tot+=parseInt(ff.elements['NomeText'+i].value,10)
		}
	}
	if(n>0)ff.NomeTextRisultato.value=(tot/n)
	else ff.NomeTextRisultato.value=''
}
</script>
<form name="NomeForm">
<input type="text" name="NomeText1" onchange="Media()">
<input type="text" name="NomeText2" onchange="Media()">
<input type="text" name="NomeText3" onchange="Media()">
<input type="text" name="NomeText4" onchange="Media()">
<hr>
<input type="text" name="NomeTextRisultato" readonly="true">
</form>